At the museum, we have an admission fee of $5.00 per person. We also have a family rate of $10.00. This was intended to give families with small children a bit of a break - we're in Utah, there are a lot of Mormons, and then tend to have a lot of kids. If it cost Mom and Dad $25.00 to bring their three children to the museum - well, we'd get a lot fewer visitors, I think it's safe to assume.
But we have had a lot of adults try and bend the rule to apply to their extended family, or their adult children. I mean, if a parent tries to argue that their college age son and his girlfriend count as family, I might bend the rule for them, because I know the college student might still be supported by his mom and dad while he's in school. But when you've got a guy in his sixties trying to argue that both his family (his wife and his forty-year-old son and his wife) and his brother's family (another two or three people) should all count as one family admission - NO. Not only are you and your brother going to have to pay separately, but the rule was not intended for adult children, so your son and his wife have to pay separately, too. I ain't budging on that one, because that is not how the rule was meant to be applied.
I noticed that my coworker simply tells people that adults are $5.00 each, and children are free. I think I'm just going to switch to that - it seems to make things much, much easier!